I don't know if it's the pressure to continue to support a successful title that's stressing the developer out or if, as an artist, he just isn't interested in associating himself with the title as much ("hey it's the 'Flappy Bird' guy!") for whatever reason. For someone being so direct, he also feels a bit guarded.
I guess he wants a much quieter existence? The money he's made is certainly going to take the pressure off of him as far as his life is concerned.